Forum der Vereinigung der Sternfreunde

Hello all! My comparison between the two most popular off-the-shelf Echelle spectrographs has been accepted and now published by PASP (Publications of the Astronomical Society of the Pacific). The paper can be found at
http://iopscience.iop.org/article/10.10 ... 15001/meta
or
http://stsci.de/pdf/eversberg2016b.pdf.
This paper should be of some interest for the amateur and professional community because the two instruments are already in use at many observatories.
Cherio, Thomas

Hi Thomas,

In your conclusions you quote an estimated accuracy of 250m/s for RV
measurements using the eShel. I am not clear if this absolute accuracy (eg
referenced to measurements of RV standards), precision ( ie repeatability)
or the stability of the instrument (which can to some extent be overcome by
frequent arc measurements). As a comparison Buil demonstrated
significantly higher precision over extended periods of typically ~50m/s
using the eShel during his measurement of 4 exoplanets.

http://www.astrosurf.com/buil/extrasolar/exo1.png
from
http://www.astrosurf.com/buil/extrasolar/obs.htm

Cheers
Robin


-----Original Message-----
From: Thomas Eversberg
Sent: Monday, September 19, 2016 3:30 PM
To: fg-spek-admin@vdsastro.de
Subject: [fg spektroskopie] New Echelle article

Link zum neuen Beitrag: http://forum.vdsastro.de/viewtopic.php?t=4479#p28096

Hello all! My comparison between the two most popular off-the-shelf
Echelle spectrographs has been accepted and now published by PASP
(Publications of the Astronomical Society of the Pacific). The paper
can be found at
http://iopscience.iop.org/article/10.10 ... 15001/meta
or
http://stsci.de/pdf/eversberg2016b.pdf.
This paper should be of some interest for the amateur and
professional community because the two instruments are already in use
at many observatories.
Cherio, Thomas

Hi Robin!

On page 4 I mention why I consider 250m/s as the eShel accuracy („the nightly scattering of the measured radial velocities“).

Your second point: Of course, I know Buil’s measurements. However, basically I had three reasons not to take these measurements into account.

First, I was not too enthusiastic to take example measurements by the instrument designer himself and a representative of the distributing company. Buil published his results in an unrefereed text on a website whereas the Kozlowski/Pribula results already passed a referee. I presume that my referee would have some objections if I would try to use website results from the company for my comparison.

Second, eShel is designed for „plug-and-play“, hence, a reasonable comparison should be performed under „user knowledge conditions“ without „designer’s knowledge“. According to my information 250m/s are closer to repeatability then Buil’s extreme. Only recently Berthold Stober confirmed the difficulties to reach eShel accuracies below 1 km/s.

And third, if I would ask the eShel designers I also would have to ask for Baches results from CAOS group and/or the respective distributor (probably again with world-class results). I needed independent observers and unsuccessfully asked in our forum. Capable group members unfortunately believed to need a lab for doing it.

It might well be that some readers disagree with my approach. But unfortunately I can’t take them into consideration as long as they don’t go into discussion or write their own publication.
Cheers, Thomas

Hi Thomas,

Thanks for the clarification. The published results look to be a reasonable
and a fair comparison of the intrinsic precision and stability of the
instruments. A skilled observer who understands the limitations of the
instrument and how to minimise their impact will do significantly better in
practise though as I have found from personal experience with the LHIRES
which has similar stability problems to those of the BACHES.

Cheers
Robin

-----Original Message-----
From: Thomas Eversberg
Sent: Monday, September 19, 2016 6:45 PM
To: fg-spek-admin@vdsastro.de
Subject: [fg spektroskopie] Re: New Echelle article

Link zum neuen Beitrag: http://forum.vdsastro.de/viewtopic.php?t=4479#p28099

Hi Robin!

On page 4 I mention why I consider 250m/s as the eShel accuracy („the
nightly scattering of the measured radial velocities“).

Your second point: Of course, I know Buil’s measurements. However,
basically I had three reasons not to take these measurements into
account.

First, I was not too enthusiastic to take example measurements by the
instrument designer himself and a representative of the distributing
company. Buil published his results in an unrefereed text on a website
whereas the Kozlowski/Pribula results already passed a referee. I
presume that my referee would have some objections if I would try to
use website results from the company for my comparison.

Second, eShel is designed for „plug-and-play“, hence, a reasonable
comparison should be performed under „user knowledge conditions“
without „designer’s knowledge“. According to my information
250m/s are closer to repeatability then Buil’s extreme. Only recently
Berthold Stober confirmed the difficulties to reach eShel accuracies
below 1 km/s.

And third, if I would ask the eShel designers I also would have to ask
for Baches results from CAOS group and/or the respective distributor
(probably again with world-class results). I needed independent
observers and unsuccessfully asked in our forum. Capable group members
unfortunately believed to need a lab for doing it.

It might well be that some readers disagree with my approach. But
unfortunately I can’t take them into consideration as long as they
don’t go into discussion or write their own publication.
Cheers, Thomas

Hi Robin! That is probably all correct. The more you invest above the basic design the better are the results (until a certain physical threshold). However, that are influences one can not measure and that are efforts above the initial design and price. I mean, we can fully re-design BACHES, LHIRES etc. But that would be no off-the-shelf instruments anymore. Better is to test the manufacturers claims and compare them with other instruments (Baader claimed an accuracy of 50 m/s for BACHES). If we blow-up such tests (optical efficiency, potential user improvements etc.) we might confuse ourself and obtain any result without really testing the initial instrument (if we then can test it at all). I believe this is not a good strategy.
I hope that my paper is also a motivation for other instrument tests. Otherwise we fully depend on the manufacturers information and pay a lot for nothing.
Cheers, Thomas

Hi Thomas,

The investment is mainly in time understanding the instrument and
developing the appropriate observing procedures, not in money. For example
long term calibration drifts are not a problem provided they are longer than
the exposure which is typically a few tens of minutes at most. The long term
drift is removed by measuring arcs between each exposure. Only changes
within this time will limit the precision of RV measurement. Thus in
practise the total instability of 0.25km/s you estimate for the eShel will
easily be improved on in practise. In your paper you refer to (and then
dismiss as unreliable due to inadequate documentation) a short term
stability of 50 m/s for the eShel . This is however consistent with the
uncertainties Buil et al found when measuring the RV of a number of stars
hosting exoplanets.

Once you try to measure RV to a precision better than 1km/s though, the
stability of the spectrograph as measured using the calibration lamp is only
one of the issues. For example the slit spectrograph is at a severe
disadvantage compared with the fibre feed version as the precise measured
location of the line in the star spectrum depends critically on the exact
position of the target on the slit. Even a shift of a fraction of a pixel
can (and does) produce significant errors as documented recently here for
example.
http://spectro-aras.com/forum/viewtopic ... 5692#p5637

Cheers
Robin

-----Original Message-----
From: Thomas Eversberg
Sent: Friday, September 23, 2016 1:39 PM
To: fg-spek-admin@vdsastro.de
Subject: [fg spektroskopie] Re: New Echelle article

Link zum neuen Beitrag: http://forum.vdsastro.de/viewtopic.php?t=4479#p28115

Hi Robin!
That is probably all correct. The more you invest above the basic
design the better are the results (until a certain physical threshold).
However, that are influences one can not measure and that are efforts
above the initial design and price. I mean, we can fully re-design
BACHES, LHIRES etc. But that would be no off-the-shelf instruments
anymore. Better is to test the manufacturers claims and compare them
with other instruments. If we blow-up such tests (optical efficiency,
potential user improvements etc.) we might confuse ourself and obtain
any result without really testing the initial instrument (if we then
can test it at all). I believe this is not a good strategy.
I hope that my paper is also a motivation for other instrument tests.
Otherwise we fully depend on the manufacturers information and pay a
lot for nothing.
Cheers, Thomas

Hi Robin! I am not sure if this is really correct for all spectrographs. As far as I understand, Buil for instance used a mimic against fiber noise. And for BACHES I assume some necessary action agains flexure.
I am not so sure! The same is claimed by Kozłowski et al. (2014) but they describe neither those shifts that can be compensated nor the corresponding procedure (see my page 4). Figure 2 shows that even the calibration spectra stochastically vary by up to 6km/s within one hour. Hence, potentiaL trouble can quickly be underestimated.
I am not sure if this can be easily done considering calibration arc variability of about 100m/s within 15 minutes (see figure 7).
I rest my case and already explained my reasons.
I know that.

Robin, I appreciate your comments and it might be that eShel measurements are even better. But as long as there is no repeated output from others I consider Buil's results as expert output not regularly achievable by normal users. If you disagree you should collect such measurements, describe the procedures and write a paper for PASP.

Cheers, Thomas

Hi,

this article has a very basic problem:

The author does not has any experience with either one of the instruments. (Don't take is personally, Thomas, but that's fact.)

Hence, I understand it as a summary of published (refereed) studies - which can't be considered as a final judgement of these two instruments.

best wishes,
Daniel

_________________
Daniel P. Sablowski

https://www.kunstmann.de/buch/axel_hack ... 42000/t-2/
https://dpsablowski.wordpress.com
https://github.com/DPSablowski
https://www.aip.de/mitglieder/dsablowski/

Daniel, your statement means that one can only compare the two spectrographs if one is practically familiar with both of them. That's probably the reason why you believe that only laboratory tests should be applied. A strange argument. Otherwise we would still wait for such information (luckily my referee agrees with me).
Again, these devices are off-the-shelf instruments ready to use. For a reasonable comparison any advanced operation like opto-mechanical improvements or specific action not possible for both instruments should then not be taken into consideration (I repeat myself). And we now have at least a hint for a buyers decision. In this respect I consider my comparison as a final judgement.
Above that one can only perform an investigation for a single spectrograph. I can only motivate you, Robin and others to do that!
That's all, Thomas

Thomas,
I can't find that statement anywhere in my last post!!!

Why you are using arguments against things, I haven't said???

THAT IS STRANGE !!!

disappointed,
Daniel

_________________
Daniel P. Sablowski

https://www.kunstmann.de/buch/axel_hack ... 42000/t-2/
https://dpsablowski.wordpress.com
https://github.com/DPSablowski
https://www.aip.de/mitglieder/dsablowski/

Hello Thomas,

you wrote

" Above that one can only perform an investigation for a single
spectrograph. I can only motivate you, Robin and others to do that!"

Over the past 10 years I have owned (and still own and use) 3 commercial
spectrographs, one of which was originally developed by a team of amateurs
and another which I developed and is owned by over 4000 amateurs worldwide.
During this time I have continuously reviewed the performance of the
equipment I use and published the results. I have also made simple
modifications to improve them and developed techniques to get the best out
of them, some of which have been taken up commercially by the manufacturers.
I have made all this information, along with the spectra I have taken,
freely available to anyone who is interested. I typically answer several
queries every day from amateurs interested in spectroscopy and offer
practical advice based on my first hand experience using these spectrographs
to produce research quality data.
Daniel produces cost effective designs for spectrographs for amateur use and
Christian Buil has spent over 20 years developing the hardware, software and
techniques which has allowed amateurs worldwide to achieve results beyond
what anyone could have dreamed possible. How about you Thomas ?

Robin

I don't do that, Daniel! Here in the forum I repeatedly asked to perform joint tests and it was you who argued against it. Your argument was missing laboratory possibilities. I do not want to dissapoint you. So, better we stop this discussion.
Thomas

I try to bring things forward, as Buil, Daniel and you. And I appreciate all your expertise. Unfortunately I do not have such ressources. But that still does allow me to put my arguments into the basket, right? I try it with my modest knowledge, read texts and possible write papers for further discussion. I am very surprised about your firm reaction on my paper in which I do not attack anybody. Too sad!
As for Daniel, we better stop here. Thomas

Thomas
I know what I've written here:

http://forum.vdsastro.de/viewtopic.php?f=2&t=4401

About efficiency measurements, which is different to stability studies, of course! Reliable and comparable measurements of efficiency is only possible in the lab in a controlled environment and stable light sources etc.
That's out of context !!!


Daniel

_________________
Daniel P. Sablowski

https://www.kunstmann.de/buch/axel_hack ... 42000/t-2/
https://dpsablowski.wordpress.com
https://github.com/DPSablowski
https://www.aip.de/mitglieder/dsablowski/

Ah, you found it. I believe this is not out of context, Daniel. Initially and much earlier I asked to perform a joint test. Already then you wrote that a lab is stricktly necessary. I wanted to define the test approach (including efficiency measurements or not) in a team (I like to work in teams, then I learn from others). That was unsuccessful and finally I decided to try it by myself using published results from Kozlowski and Pribula (as I would have done with results from amateurs). This alternative was certainly not as good as it could have been in a ARAS/SASER/VdS group. I described my approach in the final paper so that the limitations are clear. That cost me more than a year until publication (reading, exchange with Kozlowski and Pribula, interpretation, writing, referee discussions). Instead of discussing these results and the paper drawbacks (which I know) here in the forum, you and Robin come with something like how it could be. I tried to explain my considerations but at the very end all becomes personal, i..e. the messenger (me) is attacked in person. This is somewhat frustrating and not motivating for others in this forum. I apologize if my response seems offensive, I do not question the expertise of Buil, Robin, you or somebody else, of course. I would only appreciate if the same is valid for me.

Cheers, Thomas